On-stream cleaning of wood chip digesters using chelating agents



United States Patent US. Cl. 162-38 11 Claims ABSTRACT OF THE DISCLOSURE A continuous pulp digester, such as a Kamyr digester, using hot, highly alkaline aqueous pulping liquor under pressure and having one or more heating-mixing cycles carried out continuously in the upper half of the digester, which has accumulated substantial deposits of hardness in the heating-mixing cycle equipment substantially restricting the flow of liquor through such equipment, is restored to proper operation by continuously feeding an aqueous solution of chelating agent into the digester along with the white liquor in suflicient concentration and for a long enough time to substantially remove the hardness deposits from the heating-mixing section. Such treatment is carried out while the digester remains on-stream although it is generally preferred to reduce the throughput of the digester to about one-half during the treatment period, thus increasing the residence time of the chelating agent.

The invention relates to the cleaning of wood chip digesters and more particularly relates to the cleaning of continuous wood chip digesters, such as the Kamyr digesters, without taking such digesters out of service.

In modern pulping operations wood chips are often digested in a continuous process employing a highly alkaline aqueous liquor. The process may be the soda process, employing caustic soda solution and a little sodium sulfide, or it may be the kraft process using an aqueous solution made up mainly from sodium sulfate, caustic, and water. The modern digester is generally a vertical, substantially cylindrical pressurized reactor, e.g., feet in diameter and 100 feet high. Such digesters are mainly of the Kamyr or the Impco designs. Cooking liquor, called white liquor in the trade, is introduced near the top of the reactor against an operating pressure of about 110 to 120 pounds per square inch gauge. Wood chips are also introduced at the top, or if a countercurrent system is employed, the wood chips are introduced near the bottom. A mixture of pulp and liquor is drawn otf near the bottom of the reactor. Heating and mixing is effected by withdrawing pulping mixture through a screen at one or more levels in the reactor, heating the sieved mixture, hereinafter referred to as pulp liquor, and returning the heated pulp liquor to the reactor through a downcomer to about the level of the point of withdrawal and spaced apart therefrom. Typically, two such heating-mixing cycles are operated, each at a different vertical level of the reactor. Generally the volume of pulping liquor circulated through the heating-mixing cycle is from five to seven times the volume of cooking liquor being introduced into the digester.

Kraft cooking, continuous alkaline pulping, and digesters are further discussed in: Preparation and Treatment of Wood Pulps, J. S. Stephenson, editor, vol. I, McGraw- Hill Book Company (1950), pp. 364402, 453-57; Pulp and Paper Science and Technology, vol. I Pulp, C. E. Libby, editor, McGraw-Hill Book Company (1962), pp. 181-83; Studies in Continuous Alkaline Pulping, W. I. Nolan et al., TAPPI 34 No. 12:529-38 (1951); Experimental Development of High-Speed Continuous Alkaline Pulping, W. J. Nolan et al., Pulp and Paper Magazine of Canada 53 August: 98110 (1952); Studies in Continuous Alkaline Pulping, W. I. Nolan et al., TAPPI 35 No. 1l:5( 55 10 (1952); and Studies in Continuous Alkaline Pulping, W. J. Nolan, TAPPI 36 No. 9:406- 417 (1953).

A very serious problem has arisen in that during operation calcium, magnesium and other polyvalent metal ions originally present in the wood and in the liquor feed water deposit, primarily in the form of carbonates, on the intake screens as well as on the piping and heaters, or heat exchangers, used in the heating-mixing cycles often referred to as recirculation. Such mineral deposits of polyvalent metal compounds build up over a period of several months to a degree that they become flow restricting and the volume of cooking or pulping mixture which can be cycled becomes less than about half of the design capacity, at which point the recirculation system becomes subject to sudden and substantially complete stoppage. When recirculation flow stops, the digester must be taken out of service and cleaned. Since a single digester is generally designed to handle all the output of a single plant, the whole plant is substantially shut down until the digester is back on stream. It is therefore highly desirable to minimize or avoid the shutdown of this critical piece of apparatus.

A principal object of the invention is to provide a method of on-stream cleaning of a continuous process wood chip digester of the type which is subject to clogging or fouling by flow restricting mineral deposits.

Another object of the invention is to provide a method of periodic on-stream cleaning of a continuous wood chip digester subject to clogging or fouling by flow restricting mineral deposits whereby the digester can readily be cleaned frequently enough that the throughput does not fall materially below design capacity.

These and other objects and advantages of the method of the invention will be understood by those skilled in the art upon becoming familiar with the following description and the appended claims.

The method of the invention is based on the discovery that upon injecting a stable, high capacity chelating agent for polyvalent metal ions, which form flow restricting mineral deposits, into the heating-mixing cycle of an onstream continuous wood chip digester at a concentration greater than that required to chelate all such polyvalent metal ions in solution in the cooking mixture being cycled, at least part of the added chelating agent remains in the recirculatnig heatingmixing cycle for a substantial period of time and flow restricting mineral or hardness deposits on the heaters, downcomers, and intake screens of the cycle are substantially reduced or eliminated.

Any chelating agent for polyvalent metal ions will do providing that it is highly soluble in the hot (about 350 C.) aqueous alkaline pulping liquor, that it is stable in such solutions having an alkalinity of about pH 14 and that it has a high chelating capacity for mineral depositforming polyvalent metal ions such as calcium and magnesium ions. High capacity chelating agents for polyvalent metal ions are those capable of chelating about 0.2 to 0.4 gram of calcium (expressed as CaCO per gram of chelating agent.

Examples of alkaline stable high-capacity chelating agents for polyvalent metal ions are: ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, hydroxyethylethylenediaminetriacetic acid, and nitrilotriacetic acid. These compounds in the form of free acids are soluble in the hot alkaline pulping liquor and may be used in that form if facilities are available for injecting dry or slurried compound into the pulping liquor. Generally it is much more convenient to inject an aqueous solution of chelating agent into the pressurized system of the digester. To that end, the more suitable chelating agents should have a water solubility of at least about 10 percent by weight. The foregoing compounds are therefore employed as one of their ammonium or alkali metal salts having the requisite solubility. Usually diammonium or dimetal salts or those having a greater substitution of hydrogen are employed. Ordinarily these acids are employed in the form of their sodium salts.

In carrying out the method of the invention according to a preferred embodiment the chelating agent is dis solved in water to make at least a 10 percent by weight aqueous solution and more preferably about a 30 to 45 percent by weight solution. Less concentrated solutions may be used if desired, but the less concentrated solutions tend to unduly dilute the pulping mixture and cause greater interference with the pulping operation during the on-stream cleaning process.

The aqueous solution of chelating agent is injected into the apparatus of the heating-mixing cycle, generally by means of a T-connection to a line running from the screencovered intake inside the reactor to the heat exchangers used to heat the pulping liquor, though injection can be made at other points such as to the downcomers leading down into the reactor on the return side of the heatingmixing cycle. Usually injection is made into the cycle located in the upper cooking zone of the digester.

During the treatment, the flow of white liquor into the reactor and withdrawal of digested pulp mixture, i.e., the throughput of the digester, are preferably cut down substantially so as to decrease the ratio of throughput to cycled pulp liquor. In this way, higher concentrations of chelating agent can be built up in the heating-mixing cycle while reducing loss of chelating agent into the output of the digester. In a large digester having a throughput of 150 to 200 gallons per minute and a designed circulating capacity of 1,200 to 1,500 gallons per minute, the throughput is reduced to about 70 to 100 gallons per minute, achieving a compromise between high chelating agent concentration and substantially complete loss in production.

In a digester having a capacity of about 10 tons of pulp per hour, a 10 to 45 percent by weight aqueous solution of high capacity chelating agent is injected into the heating-mixing cycle at the rate of about 3 to 15 gallons per minute, and preferably at the rate of about 6 to 10 gallons per minute until from about 1,000 to 2,500 gallons have been used. In more general terms the injection rate is from about 1 to about gallons per minute per 500 gallons per minute recirculation flow rate, the latter being taken as design capacity.

Generally a larger total volume of chelating agent solution is used in the removal of heavier mineral deposits. To achieve substantially complete mineral deposit removal, the total quantity of chelating agent injected must be capable of chelating the entire quantity of deposit to be removed. In any event, the quantity of chelating agent may be gauged by its effect and the addition may be stopped when design capacity of liquid flow of circulating pulp liquor in the heating-mixing cycle has been substantially restored, e.g., 90 percent of design capacity, as where rather thorough cleaning is being carried out. Upon stopping the addition, inherent retention of chelating agent in the digester due to a high proportion of recycle in the heating-mixing cycle results in slow gradual loss of chelating agent from the entire system. As a consequence, there is a marked tendency shown for additional mineral deposit removal after all addition of chelating agent has ceased.

When 90 percent or greater circulating capacity has been restored, and injection of chelating agent has been stopped, throughput of the digester is raised to a normal level completing the manipulative steps of cleaning the digester while it is in service.

4 EXAMPLE 1 A Kamyr wood chip digester having a design throughput of 150 gallons per minute, a recirculation capacity in the heating-mixing cycle located in the upper cooking zone of 1,400 gallons per minute, and a pulp output of about 10 tons per hour, was clogged to the extent that recirculation capacity was only 800 gallons per minute. The flow rate of incoming white liquor, which controlled throughput, was cut to gallons per minute while maintaining production of pulp at a reduced rate, and a 38 percent by weight aqueous solution of the tetrasodium salt of ethylenediaminetetraacetic acid was injected at an average rate of 7 gallons per minute over a period of 8 hours. Injection was made in the line from the intake screens to the heat exchangers. During the addition of the aqueous solution of the tetrasodium salt of ethylenediaminetetraacetic acid the flow rate in the heating-mixing cycle increased steadily, as the system became unclogged, from a level of 800 gallons per minute to a level of 1,280 gallons per minute. Injection of 220 gallons of water followed immediately, whereupon the flow rate was found to have risen to 1,300 gallons per minute. Normal operation of the digester was then resumed 15 hours later the recirculation flow rate was found to have risen still further to the design capacity of 1,400 gallons per minute.

EXAMPLE 2 A 430 ton per day Kamyr wood chip digester having a design throughput of 150 gallons per minute and a recirculation capacity in the heating-mixing cycle located in the upper cooking zone of 1,500 gallons per minute was clogged to the extent that recirculation capacity was only 600 gallons per minute. The flow rate of incoming white liquor was cut to 70-75 gallons per minute while maintaining production of pulp at a reduced rate. Then a 38 percent by weight aqueous solution of the tetrasodium salt of ethylenediaminetetraacetic acid was injected into the heating-mixing cycle at the level of the upper cooking zone in the line leading from the intake screen to the heater. Injection was made at a rate varying from 9.5 to 12.5 gallons per minute. After about to gallons of the aqueous chelant solution had been injected, the recirculation flow rate in the upper zone heating-mixing cycle jumped from 600 to 1,150 gallons per minute. The balance of 2,000 gallons of aqueous chelant solution was added over about a 4 hour period. At the end of the injection period, the recirculation rate was 1,380 gallons per minute. The throughput of the digester was then brought back to 150 gallons per minute as normal operations were resumed. Five hours after the injection was completed the recirculation flow rate had risen still further to 1,400 gallons per minute.

If desired, the chelating agent employed may be injected in dry form into the stream of pulp liquor in the heating-mixing cycle. This may be accomplished by (1) placing the chelating agent in dry particulate form in an enclosed compartment which is provided with a communicating passage to the stream of pulp liquor and appropriate valving, (2) pressurizing the compartment and (3) screw extruding the particulate material directly into the stream of circulating pulp liquor.

While the present method of removing flow restricting deposits may be carried out on each of the recirculation means, i.e., heating-mixing cycles, of a digester, it is generally sufficient to carry out the process on the upper most recirculation means. The lower recirculation means are less subject to fouling and are generally cleared by the chelating agent which gets into the pulping mixture during cleaning of the upper recirculation means.

I claim:

1. The method of on-stream removal of flow-restricting mineral deposits from the recirculation means of a continuous Wood chip digester used in a pulping process employing a hot, highly alkaline aqueous white liquor and said recirculation means being provided for continuously Withdrawing pulping liquor from an upper cooking zone of the digester, continuously heating said withdrawn pulping liquor, and continuously returning the pulping liquor to the upper cooking zone, which comprises:

injecting an alkaline-stable high capacity chelating agent for polyvalent metal ion into the withdrawn pulping liquor in the recirculation means in sufficient amount and for a sufiicient time to substantially remove flow restricting mineral deposits from the recirculation means, said chelating agent being further characterized as being stable in hot aqueous alkaline pulping liquor having a pH of about 14, in having the capacity to chelate about 0.2 to 0.4 gram of calcium, expressed as CaCO per gram of chelating agent, and in having a water solubility of at least about percent by weight.

2. The method as in claim 1 in which the chelating agent is employed as an aqueous solution having a concentration of at least 10 percent by weight and the aqueous solution is injected at a rate of from about 1 to 5 gallons per minute per 500 gallons per minute of withdrawn pulping liquor being recirculated for a sufficient time to substantially remove flow restricting mineral deposits from the recirculation means.

3. The method as in claim 2 in which the aqueous solution of chelating agent employed has a concentration in the range of about 10 to 45 percent by weight.

4. The method as in claim 2 in which the aqueous solution of chelating agent employed has a concentration in the range of 30 to 45 percent by Weight.

5. The method as in claim 1 in which the chelating agent is a compound selected from the group consisting of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, hydroxyethylethylenediamine triacetic acid and nitrilotriacetic acid and the ammonium and alkali metal salts thereof.

6. The method as in claim 1 in which the chelating agent is the tetrasodiurn salt of ethylenediaminetetraacetic acid.

7. The method as in claim 1 in which the chelating agent is injected in the form of an aqueous slurry.

8. The method as in claim 1 in which the total amount of chelating agent injected exceeds that amount needed to chelate all of polyvalent metal ions in the flow restricting mineral deposits and completing the injection within a period of about 24 hours 9. The method as in claim 1 in'which the throughput of the digester is substantial reduced during injection of the aqueous solution of chelating agent.

10. The method as in claim 9 in which throughput of the reactor is reduced about percent by volume substantially throughout the injection period.

11. The method as in claim 1 in which the flow-restricting mineral deposit has reduced the recirculation flow to less than about /3 of design capacity and including the step of injection suflicient aqueous solution of alkalinestable high capacity chelating agent to restore the recirculation flow to at least percent of design capacity.

References Cited UNITED STATES PATENTS 2,851,355 9/1958 Battenberg et al. 162-76 X 2,920,010 1/1960 Voiret 16276 2,947,657 8/1960 Peteri 16276 3,071,504 1/1963 Dunklin et al. 162-199 3,143,458 8/1964 Charron 16276 OTHER REFERENCES Pulp and Paper Science and Technology, vol. I, 1962 McGrawHill, New York, N.Y., pp. 79 and 80.

Sequesterene, 1952, Geigy, N.Y., p. 48.

S. LEON BASHORE, Primary Examiner.

US. Cl. X.R. 

